The fuel assemblies of nuclear reactors cooled by water under pressure or boiling water are formed by combustible elements termed rods, maintained in a rigid framework so as to constitute a cluster in which the rods are disposed in parallel relation to one another.
The fuel rods are formed by sheathing tubes, generally of zirconium alloy, for example Zircaloy 4, enclosing pellets of uranium oxide or sintered plutonium stacked one on top of the other in the axial direction of the tube. The tube is sealed by plugs of zirconium alloy and the sintered fuel pellets are placed inside the tube in an inert gas atmosphere under pressure.
The framework of the assembly is generally constituted by transverse spacer grids for maintaining the rods in a regular network and by longitudinal guide tubes rigidly fixed to the spacer grids in the region of certain cells reserved for the passage of the guide tubes.
The fuel assembly is generally closed at its ends by end members fixed to the ends of the guide tubes.
Most of the elements of the framework of the fuel assemblies of water-cooled reactors are made from zirconium alloy.
The elements for regulating and stopping the nuclear reactor comprise, as the case may be, sheathed or unsheathed bars of alloyed or non-alloyed hafnium sliding in the guide tubes of the assembly.
In the nuclear reactor in operation, the fuel assemblies are subjected to various types of corrosive attack on their surface in contact with the cooling water of the reactor or on their surface in contact with the fuel.
The sheathing tubes of the fuel elements through which there is a large thermal flow and which are in contact with the cooling water only on their outer surface are principally subjected to oxidation and to the various forms of attack due to interaction between the fuel pellets, which undergo a swelling under irradiation, and the inner surface of the tube.
The elements constituting the framework, and in particular the guide tubes which are in contact with the cooling water on both sides and through which there is no large thermal flow, undergo principally a hydridation under the action of the hydrogen which is formed upon contact at high temperature between the cooling water and the zirconium constituting the principal element of the components of the assembly, in accordance with the reaction: EQU Zr+2H O.fwdarw.ZrO.sub.2 +2H.sub.2.
A part of the hydrogen formed by this reaction is fixed by the metal constituting the components of the framework, in which it may precipitate in the form of hydride.
In the ambience of the reactor, the components of the fuel assemblies of zirconium alloy are covered with a layer of zirconium in accordance with the aforementioned chemical reaction.
The layer of zirconium formed is liable to thicken in consuming the zirconium alloy, in particular in the case of the fuel sheathing tubes. As the oxide formed is a poor heat conductor, the kinetics of the corrosion will increase with the thickness of the oxide, since this kinetic is related to the metal-oxide interface temperature. The thickness of tho metal constituting these sheathing tubes may then become insufficient to ensure that the fuel element is properly maintained position in the reactor.
Furthermore, the hydrogen formed upon oxidation of the zirconium by tile cooling water is given off in contact with the metal of the tube.
In the case of the elements constituting the framework of the assembly, the layer of zirconium formed in the presence of hydrogen on these elements does not permit limiting the diffusion of the hydrogen and therefore the hydridation.
The film of natural oxide (a few nanometers) which covers the elements of hafnium may be deteriorated by the effect of the friction of these elements against one another or against their sheathing tube. It can then no longer afford protection against hydridation by the hydrogen which comes from the primary medium and passes through the stainless steel sheathing tube. This hydridation of the hafnium results in swelling of the metal which may reach 15% in volume and require premature replacement of the affected elements.
Until now, no method was known for effectively protecting the elements constituting the fuel assemblies of water-cooled nuclear reactors, composed of zirconium alloy, or generally, another passifiable alloy or metal such as hafnium and the alloys thereof.